Paul Manafort departs the federal courthouse after a hearing in Washington DC on 14 February 2018.
Photograph: Shawn Thew/EPA

The special counsel investigating possible collusion between the Trump campaign and the Kremlin has stepped up pressure on the former campaign manager Paul Manafort by filing new sealed court charges against him.

Reuters saw a court record referring to the sealed charges, which were also directed against Rick Gates, Manafort’s former business associate, but the documents provided no details on the nature of the charges.

It was not clear if the charges involved alleged bank frauds the special counsel’s office referred to in a court dispute over Manafort’s bail on Friday, in which he is said to have doctored profit and loss statements from his firm, in order to get a mortgage on a property in Virginia.

NBC News reported on Wednesday that Manafort was being investigated for having allegedly promised the president of the Chicago-based Federal Savings Bank that loaned him his mortgage a job in the Trump White House in return for $16m in home loans on his properties in Virginia, New City and the Hamptons.

The bank president, Stephen Calk, served as a Trump adviser during the campaign but did not get a cabinet post. His ties to Manafort were first reported last year by Bloomberg News.

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Manafort, who was Trump’s campaign manager for almost five months in 2016, and Gates, who was deputy campaign manager, were indicted by the special counsel, the former FBI chief Robert Mueller, in October. They face charges including conspiracy to launder money, conspiracy to defraud the United States and failure to file as foreign agents for lobbying work they did on behalf of the pro-Russian Ukrainian Party of Regions. Both have pleaded not guilty.

The new charges and the leaked details of an investigation into Manafort home loans come a day after Alex van der Zwaan, a Dutch lawyer who worked with Manafort and Gates in representing pro-Russian interests in Ukraine, admitted lying to investigators as part of a plea deal with Mueller’s office.

Gates is also reported to be negotiating a plea deal, increasing the pressure on Manafort to cooperate with the Russia investigation. Manafort is currently under house arrest and is seeking to agree bail to allow him to move freely, but Mueller’s office is disputing the value of the assets, including his various properties, arguing that they were vulnerable to foreclosure, because of what Mueller’s office says is newly uncovered evidence of “additional criminal conduct”.

The special counsel’s office declined to comment on the new court filing, lawyers and representatives for Manafort and Gates could not be immediately reached for comment.

Mueller was appointed in May 2017 to investigate Russian interference in the 2016 election campaign, possible collusion between Trump’s campaign and Moscow with Russia and whether Trump tried to obstruct the investigation. Trump has denied any collusion and has denounced the investigation as a witch-hunt.